This invention relates to drying of traveling webs and, more particularly, the drying of a newly formed paper web on a papermaking machine.
Prior dryer arrangements pertaining to double felts, impingement drying (moving hot air against the wet surface) or through air drying (blowing heated air through the web) either guide the bare web onto the dryer shell without support or sandwich it between two felts while carrying it on the dryer roll. Sometimes, a single felt is used in combination with an impingement and/or vacuum arrangement to promote removable of moisture.
All of these arrangements have deficiencies and inefficiencies which become especially troublesome when it is desired to operate at or near the fastest speed the machine is designed for. Such inefficiencies are usually manifested by the web billowing off the dryer roll surface or edge flutter, both of which contribute to web breaks, or simply a decrease in the drying rate as the web passes over the dryers. When speeds increase, the rate of drying must also increase in order to keep the web dryness at the end of the machine within predetermined limits. Double felted air impingement dryers have sometimes required a special, endless belt-like arrangement in addition to the top felt in order to keep the web from fluttering under the force of the impinging air. On configurations wherein both felts track over conventional dryer shell surfaces, web billowing is suppressed, but so is the rate of moisture removal. Furthermore, additional equipment must then be used to remove moisture from the felts in the gaps between dryer rolls. On through air drying arrangements wherein the web is carried on a single felt or belt, the web stability due to edge flutter is impaired as it must travel between dryers without support on one or both sides.
In summary, prior art arrangements have tended to sacrifice drying capacity and efficiency for speed and vice versa.